1. Field of the Invention
The present invention relates to a pneumatic tire in which a sipe is formed in a land portion of a tread surface, and is particularly useful as a studless tire.
2. Description of the Related Art
Conventionally, in a studless tire, a cut called as the sipe is formed in the land portion of a block, a rib or the like, and a traveling performance on an ice road surface having a low friction coefficient is increased on the basis of an edge effect and a drainage effect generated by the sipe. As the sipe mentioned above, there have been a linear sipe extending linearly in a sipe length direction, a waveform sipe extending like a waveform and the like, in practical use.
In this case, since a rubber is incompressible, a motion of the tread surface being in contact with the road surface becomes great in a case where a load is applied to the tire, so that an opening portion of the sipe tends to be closed. Particularly, since the tread surface tends to be moved due to its slip, the sipe is easily closed and the opening portion is narrowed, on the road surface having the low friction coefficient such as the ice road surface or the like, there is a case that the edge effect and the drainage effect to be essentially achieved by the sipe are lowered.
Accordingly, the inventor of the present invention has invented a pneumatic tire forming a sipe having a plurality of wide portions extending in a sipe length direction (refer to Japanese Unexamined Patent Application Publication No. 2007-8303). In accordance with this pneumatic tire, since a tread rubber tends to be deflected in a sipe depth direction, it is possible to absorb a deformation caused by a load applied to the tire by the wide portions, at a time when the load is applied to the tire. As a result, it is possible to inhibit an opening portion of the sipe from being narrowed so as to secure an edge effect and a drainage effect.
However, after the inventor of the present invention has conducted researches further, the following matter has been found. In other words, in the case of forming the sipe having the wide portions as mentioned above, it has been known that a ground contact pressure of a kick-out side portion 61 of a sipe 6 becomes greater than a ground contact pressure of a step-in side portion 62 conceptually shown in FIG. 11, and a ground contact pressure of a block 7 becomes uneven if a braking force is applied to the tire on the ice road surface on the basis of a tread rubber being easily deflected in a sipe depth direction. Accordingly, it can not be said that an ice braking performance (a braking performance on the ice road surface) is sufficiently achieved, and it is known that there is room for further improvement of the ice braking performance.